Centrifuge drive



May 23, 1944.

C. A OLCOTT CENTRIFUGE DRIVE Filed Nov. 8, 1940 TIME SW/TCH unsrikSWITCH m con/mar UNZOAD/NG 5, 550

TIME SECONDS INVENTOR C. A. OLCOT T BY ATTORNEY Patented May 23, 1944UNITED STATES PATENT OFFICE CENTRIFUGE DRIVE Charles A. Oloott, WestMilford, N. J.

18 Claims.

This invention-relates to centrifuges such as Application November 8,1940, Serial No. 364,777

employed in the manufacture and refining of sugar and more particularlyto methods and means for the electric driving of such apparatus.

The refining of sugar involves at one step of the process thecentrifugal separation of sugar crystals from a mass of magma throughoutwhich the crystals are dispersed. The magma is loaded into the basket ofthe centrifuge and the latter is spun at high speed to expel the fluidmaterial, leaving the crystals within to be unloaded. In a typicalinstallation in accordance with present day practice the centrifugebasket is 40 inches in diameter and at top speed its rotational velocity is approximately 1800 R. P. M. The economics of the process aresuch that the centrifuges are in almost continuous operation and specialprovisions are made to reduce to a minimum the time required to load thecasket, accelerate it to top speed, decelerate it and no load it. Theentire cycle may occupy only a few minutes. One cycle of operation mayinvolve 500 lbs. oi magma.

The purging of the crystals is more effective the greater thecentrifugal force developed dur ing the high speed operation of thecentrifuge but this is limited at least in part by the limited strengthof the basket. Centrifuges are driven most advantageously by electricmotors of the squirrel cage induction type designed with reference tothe available alternating current supply to operate at the desiredspeed. The speed of such motors is not continuously variable and, infact, the operating speed cannot readily be changed except in largeincrements dependent on the number of poles and the operating powerfrequency, and even such changes require rewiring of the machine. Forexample, a motor having twelve poles and operated on 60 cycle currentcan be wired to operate at speeds approximating 600, 900, 1200, 1860 and3600 R. P. M., these speeds corresponding respectively with 12-, 8-, 6-,4- and 2-poie connection.

In many cases the centrifuge is capable of safe operation at a speedsubstantially higher than that for which'the driving motor is wired butnot enough higher'to permit rewiring of the motor for operation at thenext higher synchronous speed. Thus if the actual operating speed isi888 R. P. M.. the basket may, for example, safely withstand 2200 R. P.M. but not 3600, the next higher synchronous speed. Heretofore inpractice where the centrifuge is directly connected to an individualdriving motor it has been necessary to'forego the advantages that wouldbe obpacity.

tained with operation at 2200 R. P. M. in such cases.

Qne of the principal objects of the present in-- vention is to improvethe effectiveness of purging in the operation of a centrifuge. Anotherobject is to provide for the efllcient operation of electrically drivencentrifuges at speeds higher than those for which their motor driveswere originally arranged or alternatively to reduce the time required tocomplete the centrifuging cycle.

In accordance with the present invention the foregoing objects and otherobjects that will appear hereinafter are attained by providing foroperation of the centrifuges on power currents of different frequencies.Current of one fixed frequency is ordinarily supplied by the main powersystem associated with the refinery, whether it be a power plantsupplying only the electrical requirements of the refinery or anelectric power network system supplying power also throughout the areain which the refinery is located. The frequency of this main system isgenerally not subject to change.

The invention in its preferred form contemplates the provision of an"auidliary power system adapted to supply power currents of a differentfrequency for selective application to the centrifuge driving motors.The auxiliary system need have but a small fraction of the powergenerating capacity of the main system and it may take the form merelyof a frequency converter set connected to and driven from the m i p w sye o supply current of the desired irequency, or it may, for example,embrace a house power generating plant of moderate ca Assuming that thecentrifuge driving alternating current motors, selected rence to thefrequency of the main powe s tern to drive the centrifuge baskets at apredetermined speed, and that operation at a higher speed short of thenext higher synchronous is desired, the invention further con= templatessuch choice of the frequency of the auxiliary system in relation to thedesign of the driving motors that the latter when driven from theauxiliary system operate at the desired higher speed. In the simplestand preferred case motors the frequency of the auxiliary system ishigher the power capacity required of the auxiliarysystem is minimizedby drawing on the main power system for the relatively high powerrequired to accelerate the centrifuge from loadoperation at the higherultimate or synchronous speed obtainable on connection to that system.

Bearing in mind that if the auxiliary system is of small capacity or ifin any case it is energized by the conversion of power from the mainsystem, its efiiciency of operation may be relatively low and the energytaken from it comparatively costly, it may be' noted that the featurelast described tends to place the greater part of the energy load, aswell as the power load, on the presumably highly efficient main system.

In accordance with another feature of the invention, the regenerativedeceleration or braking of the centrifuge that ordinarily followsimmediately the period of operation at maximum speed is effected byautomatically reconnecting the centrifuge driving motor to the mainsystem rather than by allowing the motor to regenerate into theauxiliary system to which it was last connected. One of the severaladvantages attendant on this feature is that the efficiency of energyreconversion is relatively high, for the dynamic power of the centrifugeis returned directly to the main power system rather than eitherindirectly through a frequency changer or into the auxiliary powersystem.

The nature of the present invention and its various features, objectsand advantages will ap-- pear more fully from a consideration of thefollowing description of the illustrative embodi-' ments shown in theaccompanying drawing. in the drawing,

Fig. 1 illustrates schematically a system for the driving of centrifugesin accordance with the invention;

Fig. 2 shows a modification thereof;

Fig.3 comprises curve diagrams to which reference will be made in thedescription of Figs. 1 and 2.

Referring more particularly now to Fig. 1 there is shown in outline thepower supply and control circuits for the operation of all of thecentrifuges in a given refinery. The apparatus and circuits individualto one centrifuge are shown completely in schematic form and it willreadily appear how the system may be extended to accommodate any numberof centrifuges in the same plant.

The centrifuge is represented diagrammatically by the basket I which isdriven by its individual centrifuge motor 2. The main power supplysystem, which as indicated hereinbefore may be local to the plant orpart of a power supply network, is represented by the bus-bars 3 and ispresumed to be 3-phase. The auxiliary power supply sys tern isrepresented by a frequency changer i the input terminals of which areconnected to busbars 3 and the output terminals of which are connectedto the 3-phase bus-bar system 5. It may be assumed for specific examplethat the main power system operates at 60 cycles, that the frequencychanger produces power currents of 80 cycles, and that the driving motor2 when connected to a cycle source hasa' synchronous speed of 1800 R. P.M. Throughout this specification it should be understood that operationat a given synchronous speed may not actually be -achieved inasmuch asthere may be with certain types of motors commonly used a certainamount.

of slip. The latter may be, for example, about 5 per cent. Thus thespecified synchronous speed of 1800 R. P. M. may correspond to anactual,

speed Of 1710 R. P. M.

The connection of the driving motor 2 to one or the other of the bus-barsystems 3 and 5 may .be effected by means of a biased multi-elementswitch 8 that is operated by means of an electromagnet 8. In the normalposition of switch 6 bus-bar system 3 is connected through contactelements I to the contactors in which are in the input leads to themotor 2. The contactors III are controlled by master switch II to makeor break the power circuit leading to the motor. In the alternativeposition of switch 6 contact elements 8 are closed to connect bus-barsystem 5 to the motor circuit contactcrs i0.

Switch magnet 9, which may be energized from any suitable source, isoperated through the intermedlary of a time switch l2 at a predeterminedtime after, and in response to, closure of master switch II. There maybe provided also a second time switch l8 so associated with magnet 9 andtime switch l2 as to cause magnet 8 to be deenergized, thereforereleasing switch 6, at a predetermined time after the operation of timeswitch If. In view of the fact that time switches and their arrangementfor performing the functions indicated are well known in the art adetailed description of them is considered superfluous.

From the foregoing description of Fig. 1 it should now be understoodthat on manual closure of master switch ii, e0 cycle power is applied tothe centrifuge driving motor 2, that after an interval fixed by timer I2switch 6 operates to disconnect the motor from the 60 cycle system andto connect it to the auxiliary cycle system, and that after a furtherinterval fixed by timer ii the motor is reconnected to the 60 cyclesystem.

The operationof the Fig. 1 system as above described may be statedbriefly as follows. It may be assumed that initially the centrifuge iseither at rest or rotating at the slow speed sometimes used in loadingthe basket. On closure of master switch il power is supplied to themotor from the 60 cycle system thereby causing the centrifuge toaccelerate rapidly. This initial acceleration may be effected in two ormore stages if desired, in accordance with the usual practice, with themotor poles being automatically changed to correspond with successivelyhigher synchronou speeds. For example, the motor may be a twelve-poletwo-speed motor having a low synchronous speedof 900 R. P. M. and a highsynchronous speed of 1800 R. P. M. with automatic change over from theone to the other as the motor gains speed.

The time required to accelerate the centrifuge to a predetermined speedwill be substantially the same from one centrifuging cycle to another,hence it may be said that the time switch If! operates to switch thedriving motor to the 80 cycle system after a predetermined speed isreached. Preferably the timer i2 is so adjusted that the switching takesplace only after the initial acceleration has brought the centrifuge toa speed at least half the top speed tliat'is to be attained onconnectlonto the 80 cycle system so that substantial benefit is derivedfrom the apportionment of the energy and power loads on the two powersystems. For maximum benefit of this character the centrifuge may beallowed to accelerate, while connected to the 60 cycle system, until itreaches substantially top speed for that frequency and thereuponimmediately tranferred to the 80 cycle system. It is preferred,

however, to obtain more nearly constant, continuous acceleration and forthis objective the transfer to the 80 cycle system may advanta-' geouslybe made, in the illustrative example herein disclosed, at a speed ofabout 1600 R. P, M.

On switching the driving motor to the 80 cycle system the centrifugeresumes or continues acceleration to a still higher speed. Since thesynchronous speeds are proportional to frequency the centrifuge in thiscase would tend to approach a synchronous speed of 2400 R. P. M. Excesswindage losses, however, increase the slip and would hold the actual topspeed to about 2200 R. P. M. After operation at this speed for anypredetermined length of time (chosen with regard to the particular typeof magma being treated),

the motor 2 is switched by operation of the timer ii to the 60 cyclepower supply system. The motor 2 then operates as a generator andreturns power to the 60 cycle system, thereby being decelerated.Preferably, and in accordance with the prevailing practice, thisregenerative braking is effected with the motor poles automatically,switched over to low speed connection, and the 'speed may be therebyreduced to 1000 R. P. M. for example. Further deceleration and completestoppage may be obtained by the subsequent automatic application of amechanical brake, and any of the variety of devices that are in currentuse may be employed for this purpose.

I find it desirable in any case to have the voltages of the two powersupply systems at least approximately proportional to their respectivefrequencies. Thus if the main power system is a 440-volt 60 cyclesystem, the auxiliary system may be a 585-volt 80 cycle system. It willbe understood, then, that when the regene ative bralring begins theelectromotive force developed in the motor is considerably higher thanthe voltage of the power system to which it is then connected, which isa condition contributing to rapid deceleration.

The sequence of operations in the centrifuge cycle may be betterunderstood by reference to Fig. 3. The solid line curve in Fig. 3 showshow the rotational velocity of the centrifuge in R. P. M. variesthroughout the centrifuging cycle in a typical case. While connected tothe 60 cycle system, the centrifuge is accelerated in 85 sec-= onds to aspeed of about 1600 R. P. M., which is only slightly less than theactual maximum speed which might ultimately be obtained as indicated bythe dotted branch curve. At this point the driving motor is transferredto the 80 cycle system whereupon the centrifuge is accelerated to itsultimate speed of approximately 2200 R. P. M. and maintained at thatspeed for a period of 32 seconds. At the end of this period of highspeed operation regenerative braking on the 60 cycle power system beginsand the speed is rapid- 1y reduced to about 1000 R. P. M. At this pointa mechanical brake is applied to bring the centrifuge to rest or to alow speed su table for the unloading operation. The complete cycle inthe example illustrated requires but 360 seconds and it is immediatelyfollowed by the next cycle.

Brief consideration of the dotted line curve in Fig. 3 will fac litatean understanding of 2. This curve shows how the input current to thedriving motor varies while the motor is connected to receive power fromthe 60 cycle system. The important point to observe is that after aninitial surge to a high value the input current drops off rapidly as thecentrifuge gains speed and that there is then a fixed relation betweenthe rotational velocity and the input current. In accordance with amodification of Fi 1, use is made of the fact that when the centrifugehas accelerated to the point where it is to be transferred to the cyclepower supply, the input current to the driving motor is of apredetermined value such as indicated in Fig. 3. Means are provided,then, responsive when the input current drops to the predeterminedvalue, to operate switch 6 in lieu of timer l2.

As shown in Fig. 2 the alternative means for operating switch 0 maycomprise a current limit relay arrangement II in the power leads tomotor 2. After closure of switch II the relay arrangement It operateswhen the input current drops to the predetermined value. If desired, atimer may be provided also to perform the function of timer II in Fig.i. The relay arrangement 15 can com prise a contact making ammeter suchas that shown in Fig. l of Patent 2,189,846, issued Feb. 13, 1941), toR. L Z. Valtat, the needle of the meter being adapted to make contactwith the contact member when the current decreases to 0 amperes.Obviously, the meter is not ener for the period of time required for theInc v to accelerate, its energization being delayed, p3 any suitablemeans such as the relay timing circuit shown in Patent 2,182,637, issuedDec. 5, 1939, to N. E. Marbury, until the current is decreasing or, inother words, until the motor is oprating on the descending portion ofthe dotted curve shown in Fig. 3.

Although the present invention has been described in terms of specificembodiments particularly adapted for present day refinery practice, itwill readily be understood by those skilled in the art that theinvention is susceptible of embodiment in various other forms within thespirit and scope of the appended claims.

What is claimed is 1. In combination with a centrifuge, an alterhatingcurrent driving motor for said centrifuge, a source or low frequencypower currents and a source of high frequency power currents, said highfrequency being at least five percent greater than but less than twicethe low frequency, means for connecting said motor to said low frequencysource for acceleration of said centrifuge, and means automaticallyoperative after a predeter mined period of acceleration for switchingsaid motor from said low frequency source to said high frequency sourceto continue the acceleration of said centr 2. The comhinat n of elementsas in claim 1 and being furti characterized in that the voltaid 'ces areat least approximately tion to their respective frequencies.

.ihination in a system for the operaa centrifuge, an alternating currentdriv-. irig motor for said centrifuge, a source of low "frequency powercurrents and a source of high iireauency currents, said low frequencybeing at least half said high frequency and said high freduency being atleast five percent greater than said low frequency, means for connectingsaid motor to said low frequency source for acceleration of saidcentrifuge, and timing means for switching said motor from said lowfrequency source to said high frequency source after a predeterminedperiod of acceleration.

4. In combination with a centrifuge, an alterhating current drivingmotor for said centrifuge, a source of low frequency power currents anda source of frequency power currents, said high frequency being at leastfive percent greater than but less than twice the low frequency, meansfor connecting said motor to said low frequency source for accelerationof said centrifuge, and means, including apparatus actuated when saidmotor has reached a speed which is at least one half of the runningspeed at said high frequency,

for switching said motor from said low frequency source to said highfrequency source.

5. The method of operating a direct-connected, electrically-drivencentrifugal machine which comprises accelerating said machine withalterhating current of a first frequency to a predetermined speed whichis at least one half of the running speed of said machine at said firstfrequency and successively, continuously accelerating said machine to ahigher speed with alternating current of a, higher frequency, saidhigher frequency being at least five percent greater than but less thantwice the first frequency. a

6. The method of operating a direct-connected, electrically-drivenmachine which comprises continuously accelerating said machine, firstfor a predetermined time interval with alternating current of onefrequency and then with alternating current of a higher frequency thatis at least five percent greater than and less than twice the firstfrequency, said predetermined time interval being long enough to bringsaid machine up to a speed which is at least one half of the full speedof said machine at said first frequency.

7. In combination with a plurality of centrifugal machines each havingan individual alternating current electric drive, a first system forsupplying power currents of a certain frequency, a second system ofcomparatively low power capacity for supplying power currents of ahigher frequency that is at least five percent greater than and lessthan twice said certain frequency, means for individually connectingsaid machines to said first system for preliminary acceleration to atleast half of full speed, means for individu= ally connecting saidmachines to said second sys tem for further acceleration to andoperation at full speed, and means for individually connecting saidmachines to said first system for regenerative braking.

8. The method of operating a direct-connected electrically drivencentrifugal machine which comprises initially accelerating said machinewith alternating driving current of a first fre quency and successively,continuously accelerating said machine with alternating driving cur-,rent of a second frequency which is at least five percent greater thanand less than twice the first frequency, the synchronous speedapproached during the initial acceleration being at least half thesynchronous speed approached during the subsequent acceleration.

9. A method in accordance with claim 8 in which said initialacceleration is continued until a speed of the order of thefirst-mentioned synchronous speed is reached.

10. In combination with a sugar centrifuge or the like, an individualalternating current driving motor therefor, a main electric power systemsupplying current of a first fixed frequency, an auxiliary electricpower system supplying cur- ,rent of a different fixed frequency whichis at least five percent greater than and less than twice the firstfixed frequency, the power capacity of said auxiliary system being smallcompared with that of said main system, means for connecting said motorto said main system for acceleration, switching means for transferringsaid motor to xii) asserts said a system for operation at full speeddependent on the frequency of said auxiliary system, and means fordelaying the transfer to said auxiliary system until after said motorhas accel= erated to at least half of said full speed, whereby the powerrequired to accelerate to full speed is derived at least in large partfrom said main system and the comparatively small power required forfull speed operation is derived from said auxiliary system.

ii. In combination with a direct-driven centrifugal machine, analternating current driving motor therefor, a main electric power systemsupplying current of a first fixed frequency, an auxiliary electricpower system of comparatively small power capacity supplying current ofa different fixed frequency which is at least five percent greater thanand less than twice the first fixed frequency, means for connecting saidmotor to said main power system for rapid acceleration toward a firstsynchronous speed, switching means for transferring said motor to saidauxiliary power system for acceleration toward a higher synchronousspeed that is less than twice said first synchronous speed, and meansfor delaying the transfer to said auxiliary system until after saidmachine has been accelerated to substantially more than half said highersynchronous speed.

12. In combination with a sugar centrifuge, a direct-connectedindividual alternating current driving motor therefor, a 21 electricpower system supplying current of a first fixed frequency, an auxiliaryelectric power system supplying current of a higher fixed frequency thatis at least five percent greater than and less than twice said firstfrequency, the power capacity of said auxiliary system being only asmall fraction of that of said main power system, means connecting saidmotor to said main power system for acceleration toward a firstsynchronous speed, switching means operative to transfer said motor tosaid auxiliary power system for further acceleration toward a highersynchronous speed and operation at full speed, said synchronous speedsbeing proportioned to the frequencies of the currents supplied to themotor, and means for delaying the action of said switching means untilafter said motor has attained at least half of said full speed. I

13. In a system for the operation of a directdriven centrifuge, ,anelectric driving motor for said centrifuge, a main electric power systemand an auxiliary electric power system, said auxiliary system having asmall fraction of the power capacity of said main system, means foraccelcrating said motor with current supplied from said main powersystem to a speed not less than seven-tenths of full speed, andswitching means for connecting said motor to said auxiliary systerm foroperation at a full speed dependent on the frequency of the currentbeing supplied.

14. In a system for the direct electric drive of a centrifuge, analternating current driving motor, a main electric power system, anauxiliary electric power system the power capacity of which is smallcompared with that of said main system, and means for operating saidmotor through a cycle, comprising continuous acceleration to andoperation at full speed, such that the electric energy required foracceleration is at least comparable in magnitude with the energyrequired for the remainder of the cycle, said operating means includingmeans to connect said motor to said main power system for a first periodof acceleration and switching means, automatieally operative after saidmotor has attained substantially more than half of full speed, totransfer said motor to said auxiliary system for a second period ofacceleration and operation at full speed, the frequency of saidauxiliary power system being at least five percent greater than and lessthan twice that of said main power system.

15. In a system comprising a battery of centrifugal machines withindividual electric driving motors of a type such that the speed thereoftends to be proportional to the frequency of the driving currentssupplied and two electric power supply sources of substantiallydifferent frequencies and widely diil'erent power capacities, the sourceof higher frequency having the smaller power capacity, the method ofoperating said machines which comprises accelerating each of said motorsinitially with current from the source of greater power capacity towarda first synchronous speed to an attained speed at least half the fullspeed reached during the operating cycle, successively accelerating saidmotor with current from the other source toward a second synchronousspeed at least five percent greater than but not more than twice thefirst. to full speed, and continuing full speed operation with currentfrom said other source, where- 'by full speed is determined by thefrequency of said source of small power capacity and at least a largepart of the energy required to accelerate to full speed is derived fromsaid source of large power capacity.

16. In a system for the operation of sugar centrifuges with individualelectric motor drives where two electric power sources of diiferentfrequency are available and the higher frequency source is of relativelysmall power capacity compared with the source of lower frequency, themethod of effecting rapid, continuous acceleration and full speedoperation for a period com parable with the period of acceleration whichcomprises accelerating the motor with current from the power source ofgreater capacity to a speed substantially more than half of full speed,further and continuously accelerating the motor to full speed withcurrent from the power source of lesser capacity, and maintaining fullspeed operation with current from the last-mentioned source.

17. In combination with a centrifuge, an alternating current'drivingmotor for said centrifuge, a source of low frequency power currents anda source of high frequency power currents, said high frequency being atleast five percent greater than but less than twice the low frequency,means for connecting said motor to said low frequency source foracceleration of said centrifuge, and timing means initiated at the timeof the connection of said low frequency source to said motor andoperative a predetermined time thereafter to switch said motor from saidlow frequency source to said high frequency source to continue theacceleration thereof.

18. The combination of elements as in claim 17 in which saidpredetermined time interval is sufiiciently long to permit said motor toaccelerate to a speed which is at least one half of the full speed ofsaid motor when operated from said high frequency source.

CHARLES A. OLCO'I'I,

